Referring to FIG. 1, a conventional leadframe 8 applicable to integrated chip (IC) packaging includes a first surface 81, a second surface 82 opposite to the first surface 82, a sheet-like die pad (not shown), a plurality of leads 83 that are disposed around the die pad, and a plurality of through holes 84 that extend from the first surface 81 to the second surface 82 and that are disposed among the die pad and the leads 83.
Each of the leads 83 has opposite lead top and bottom surfaces 831, 832. The first surface 81 of the leadframe 8 is defined by the lead top surfaces 831 of the leads 83 and a pad top surface of the die pad, and the second surface 82 of the leadframe 8 is defined by the lead bottom surfaces 832 of the leads 83 and a pad bottom surface of the die pad that is opposite to the pad top surface. The die pad is for disposition of an IC thereon, and the leads 83 are electrically connected to the IC with gold wires using wire-bonding techniques.
Further referring to FIG. 2, the conventional leadframe 8 is made by punching a substrate 80 with a punching machine 9. The substrate 80 has first and second sides defining the first and second surfaces 81, 82 of the leadframe 8, respectively. The punching machine 9 includes a lower mold 91 formed with a cavity 911, and an upper punch 92 movably disposed over the lower mold 91. The punching of the substrate 80 is conducted by passing the substrate 80 through the punching machine 9 with the first side facing the upper punch 92, and punching the substrate 80 in a punching direction from the first side to the second side to form the leads 83 and the through holes 84 so as to obtain the structure of the leadframe 8 as mentioned above. Each of the through holes 84 has a top opening opened at the first surface 81 and surrounded by an arcuate edge 811 connected to the first surface 81, and a bottom opening opened at the second surface 82 and surrounded by a burr edge 821 protruding from the second surface 82.
However, since all of the through holes 84 are formed by punching the substrate 80 in the same punching direction, the leadframe 8 thus formed is often deformed due to stress accumulated therein, which results in an undesired bend in the leadframe 8 as shown in FIG. 3.